ABSTRACT

Peritidal and subtidal carbonates of the Pethei Group are a complex arrangement of fibrous and micritic synsedimentary precipitates, carbonate grains, and detrital micrite. Synsedimentary precipitates, present largely as stromatolitic laminae, constitute the bulk of the Pethei platform, providing about 75% of the total carbonate. They occur as stromatolitic precipitates (precipitates closely associated with stromatolitic laminae) which constitute about 60% of the rock and encrusting precipitates (precipitates outside stromatolites and between grains, i.e. cement) which constitute about 15% of the rock. The most abundant precipitates are fibrous, in the form of stromatolitic microdigitate stalks (tufa) and masses of radial fans, as well as encrusting pore-filling fans, is pachous rinds, and laminar crusts. Micrite precipitates occur as stromatolitic clotted micrite and encrusting isopachous rinds. The ratio of fibrous to micritic precipitates is 2:1.

Stromatolitic precipitates are arranged in three distinct microfabrics: (1) ubiquitous, spar-micrite couplets of fibrous and clotted micrite laminae; (2) deep-water vertical fibrous masses that precipitated over micritic precipitate cores; and (3) supratidal microdigitate crusts of branching fibrous stalks. Micritic cements are interpreted to be of biogenic origin, having precipitated somehow in association with microorganisms. Fibrous precipitates are interpreted to be largely abiotic. Since stromatolitic and encrusting precipitates are petrographicaily similar, they probably share similar origins. There is little evidence of sediment trapping and binding as a mechanism for stromatolite formation.

The abundance of synsedimentary precipitates varies with platform geometry: subtidal ramps > subtidal rimmed shelves > subtidal open shelves > peritidal rimmed shelves. While the basic building blocks of the Pethei platform, synsedimentary precipitates, carbonate grains, and detrital micrite, are the same as those in Phanerozoic platforms, their distribution and relative proportions are different. These differences reflect a Proterozoic carbonate factory which extended across the entire platform and which produced carbonate largely by in situ precipitation.